给定一个由N个正整数和一个整数K组成的数组arr [] ，任务是找到大小K的任何子序列的最大可能的偶数和。如果找不到大小为K的偶数和子序列，则打印-1 。
例子：
Input: arr[] ={4, 2, 6, 7, 8}, K = 3
Output: 18
Explanation: Subsequence having maximum even sum of size K( = 3 ) is {4, 6, 8}.
Therefore, the required output is 4 + 6 + 8 = 18.
Input: arr[] = {5, 5, 1, 1, 3}, K = 3
Output: -1
天真的方法：解决此问题的最简单方法是从给定数组生成大小为K的所有可能子序列，并打印最大可能值，甚至将给定数组的可能子序列求和。
时间复杂度： O(K * N K )
辅助空间： O(K)
高效方法：为了优化上述方法，其思想是将给定数组的所有偶数和奇数存储到两个单独的数组中，并对这两个数组进行排序。最后，使用贪婪技术来计算大小为K的最大和偶子序列。请按照以下步骤解决问题：
- 初始化一个变量，例如maxSum，以存储给定数组的子序列的最大偶数和。
- 初始化两个数组，例如Even []和Odd [] ，分别存储给定数组的所有偶数和奇数。
- 遍历给定数组，并将给定数组的所有偶数和奇数分别存储到Even []和Odd []数组中。
- 对Even []和Odd []数组进行排序。
- 初始化两个变量，例如i和j ，分别存储Even []和Odd []数组的索引。
- 遍历偶数[] ，奇数[]数组并检查以下条件：
- 如果K％2 == 1 ，则将maxSum的值增加Even [i] 。
- 否则，将maxSum的值增加max(Even [i] + Even [i – 1]，Odd [j] + Odd [j – 1]) 。
- 最后，输出maxSum的值。
下面是上述方法的实现：
C++
// C++ program to implement
// the above approach
#include
using namespace std;
// Function to find the
// maximum even sum of any
// subsequence of length K
int evenSumK(int arr[], int N, int K)
{
// If count of elements
// is less than K
if (K > N) {
return -1;
}
// Stores maximum
// even subsequence sum
int maxSum = 0;
// Stores Even numbers
vector Even;
// Stores Odd numbers
vector Odd;
// Traverse the array
for (int i = 0; i < N; i++) {
// If current element
// is an odd number
if (arr[i] % 2) {
// Insert odd number
Odd.push_back(arr[i]);
}
else {
// Insert even numbers
Even.push_back(arr[i]);
}
}
// Sort Odd[] array
sort(Odd.begin(), Odd.end());
// Sort Even[] array
sort(Even.begin(), Even.end());
// Stores current index
// Of Even[] array
int i = Even.size() - 1;
// Stores current index
// Of Odd[] array
int j = Odd.size() - 1;
while (K > 0) {
// If K is odd
if (K % 2 == 1) {
// If count of elements
// in Even[] >= 1
if (i >= 0) {
// Update maxSum
maxSum += Even[i];
// Update i
i--;
}
// If count of elements
// in Even[] array is 0.
else {
return -1;
}
// Update K
K--;
}
// If count of elements
// in Even[] and odd[] >= 2
else if (i >= 1 && j >= 1) {
if (Even[i] + Even[i - 1]
<= Odd[j] + Odd[j - 1]) {
// Update maxSum
maxSum += Odd[j] + Odd[j - 1];
// Update j.
j -= 2;
}
else {
// Update maxSum
maxSum += Even[i] + Even[i - 1];
// Update i
i -= 2;
}
// Update K
K -= 2;
}
// If count of elements
// in Even[] array >= 2.
else if (i >= 1) {
// Update maxSum
maxSum += Even[i] + Even[i - 1];
// Update i.
i -= 2;
// Update K.
K -= 2;
}
// If count of elements
// in Odd[] array >= 1
else if (j >= 1) {
// Update maxSum
maxSum += Odd[j] + Odd[j - 1];
// Update i.
j -= 2;
// Update K.
K -= 2;
}
}
return maxSum;
}
// Driver Code
int main()
{
int arr[] = { 2, 4, 10, 3, 5 };
int N = sizeof(arr) / sizeof(arr[0]);
int K = 3;
cout << evenSumK(arr, N, K);
} Java
// Java program to implement
// the above approach
import java.util.*;
class GFG {
// Function to find the
// maximum even sum of any
// subsequence of length K
static int evenSumK(int arr[], int N, int K)
{
// If count of elements
// is less than K
if (K > N) {
return -1;
}
// Stores maximum even
// subsequence sum
int maxSum = 0;
// Stores Even numbers
ArrayList Even = new ArrayList();
// Stores Odd numbers
ArrayList Odd = new ArrayList();
// Traverse the array
for (int i = 0; i < N; i++) {
// If current element
// is an odd number
if (arr[i] % 2 == 1) {
// Insert odd number
Odd.add(arr[i]);
}
else {
// Insert even numbers
Even.add(arr[i]);
}
}
// Sort Odd[] array
Collections.sort(Odd);
// Sort Even[] array
Collections.sort(Even);
// Stores current index
// Of Even[] array
int i = Even.size() - 1;
// Stores current index
// Of Odd[] array
int j = Odd.size() - 1;
while (K > 0) {
// If K is odd
if (K % 2 == 1) {
// If count of elements
// in Even[] >= 1
if (i >= 0) {
// Update maxSum
maxSum += Even.get(i);
// Update i
i--;
}
// If count of elements
// in Even[] array is 0.
else {
return -1;
}
// Update K
K--;
}
// If count of elements
// in Even[] and odd[] >= 2
else if (i >= 1 && j >= 1) {
if (Even.get(i) + Even.get(i - 1)
<= Odd.get(j) + Odd.get(j - 1)) {
// Update maxSum
maxSum += Odd.get(j) + Odd.get(j - 1);
// Update j
j -= 2;
}
else {
// Update maxSum
maxSum += Even.get(i) + Even.get(i - 1);
// Update i
i -= 2;
}
// Update K
K -= 2;
}
// If count of elements
// in Even[] array >= 2.
else if (i >= 1) {
// Update maxSum
maxSum += Even.get(i) + Even.get(i - 1);
// Update i
i -= 2;
// Update K
K -= 2;
}
// If count of elements
// in Odd[] array >= 1
else if (j >= 1) {
// Update maxSum
maxSum += Odd.get(j) + Odd.get(j - 1);
// Update i.
j -= 2;
// Update K.
K -= 2;
}
}
return maxSum;
}
// Driver code
public static void main(String[] args)
{
int arr[] = { 2, 4, 10, 3, 5 };
int N = arr.length;
int K = 3;
System.out.println(evenSumK(arr, N, K));
}
}
// This code is contributed by offbeat Python3
# Python3 program to implement
# the above approach
# Function to find the
# maximum even sum of any
# subsequence of length K
def evenSumK(arr, N, K):
# If count of elements
# is less than K
if (K > N):
return -1
# Stores maximum
# even subsequence sum
maxSum = 0
# Stores Even numbers
Even = []
# Stores Odd numbers
Odd = []
# Traverse the array
for i in range(N):
# If current element
# is an odd number
if (arr[i] % 2):
# Insert odd number
Odd.append(arr[i])
else:
# Insert even numbers
Even.append(arr[i])
# Sort Odd[] array
Odd.sort(reverse=False)
# Sort Even[] array
Even.sort(reverse=False)
# Stores current index
# Of Even[] array
i = len(Even) - 1
# Stores current index
# Of Odd[] array
j = len(Odd) - 1
while (K > 0):
# If K is odd
if (K % 2 == 1):
# If count of elements
# in Even[] >= 1
if (i >= 0):
# Update maxSum
maxSum += Even[i]
# Update i
i -= 1
# If count of elements
# in Even[] array is 0.
else:
return -1
# Update K
K -= 1
# If count of elements
# in Even[] and odd[] >= 2
elif (i >= 1 and j >= 1):
if (Even[i] + Even[i - 1] <=
Odd[j] + Odd[j - 1]):
# Update maxSum
maxSum += Odd[j] + Odd[j - 1]
# Update j.
j -= 2
else:
# Update maxSum
maxSum += Even[i] + Even[i - 1]
# Update i
i -= 2
# Update K
K -= 2
# If count of elements
# in Even[] array >= 2.
elif (i >= 1):
# Update maxSum
maxSum += Even[i] + Even[i - 1]
# Update i.
i -= 2
# Update K.
K -= 2
# If count of elements
# in Odd[] array >= 2
elif (j >= 1):
# Update maxSum
maxSum += Odd[j] + Odd[j - 1]
# Update i.
j -= 2
# Update K.
K -= 2
return maxSum
# Driver Code
if __name__ == '__main__':
arr = [2, 4, 10, 3, 5]
N = len(arr)
K = 3
print(evenSumK(arr, N, K))
# This code is contributed by ipg2016107C#
// C# program to implement
// the above approach
using System;
using System.Collections.Generic;
class GFG {
// Function to find the
// maximum even sum of any
// subsequence of length K
static int evenSumK(int[] arr, int N, int K)
{
// If count of elements
// is less than K
if (K > N) {
return -1;
}
// Stores maximum even
// subsequence sum
int maxSum = 0;
// Stores Even numbers
List Even = new List();
// Stores Odd numbers
List Odd = new List();
// Traverse the array
for (int l = 0; l < N; l++) {
// If current element
// is an odd number
if (arr[l] % 2 == 1) {
// Insert odd number
Odd.Add(arr[l]);
}
else {
// Insert even numbers
Even.Add(arr[l]);
}
}
// Sort Odd[] array
Odd.Sort();
// Sort Even[] array
Even.Sort();
// Stores current index
// Of Even[] array
int i = Even.Count - 1;
// Stores current index
// Of Odd[] array
int j = Odd.Count - 1;
while (K > 0) {
// If K is odd
if (K % 2 == 1) {
// If count of elements
// in Even[] >= 1
if (i >= 0) {
// Update maxSum
maxSum += Even[i];
// Update i
i--;
}
// If count of elements
// in Even[] array is 0.
else {
return -1;
}
// Update K
K--;
}
// If count of elements
// in Even[] and odd[] >= 2
else if (i >= 1 && j >= 1) {
if (Even[i] + Even[i - 1]
<= Odd[j] + Odd[j - 1]) {
// Update maxSum
maxSum += Odd[j] + Odd[j - 1];
// Update j
j -= 2;
}
else {
// Update maxSum
maxSum += Even[i] + Even[i - 1];
// Update i
i -= 2;
}
// Update K
K -= 2;
}
// If count of elements
// in Even[] array >= 2.
else if (i >= 1) {
// Update maxSum
maxSum += Even[i] + Even[i - 1];
// Update i
i -= 2;
// Update K
K -= 2;
}
// If count of elements
// in Odd[] array >= 2
else if (j >= 1) {
// Update maxSum
maxSum += Odd[j] + Odd[j - 1];
// Update i.
j -= 2;
// Update K.
K -= 2;
}
}
return maxSum;
}
// Driver code
public static void Main(String[] args)
{
int[] arr = { 2, 4, 10, 3, 5 };
int N = arr.Length;
int K = 3;
Console.WriteLine(evenSumK(arr, N, K));
}
}
// This code is contributed by gauravrajput1 输出
18时间复杂度： O(N * log N)
辅助空间： O(N)